Technical Field
The present disclosure relates to a filter, and more particularly, to a common-mod filter and a circuit structure including the common-mode filter.
Description of Related Art
As human demands for electronic devices become more sophisticated, the feature of high transmission efficiency has become the basic requirement of a new generation of electronic devices. However, in meeting the needs of high efficient transmission, it certainly caused some unexpected circumstances. For example, if the transmission frequency of a signal is continuously increased, the electromagnetic wave is also easy to spread through mediums, resulting in the so-called electromagnetic interference (EMI). The EMI is harmful to the human body and may also cause the produced circuit layout not to pass some security certification specifications.
Under the circumstances, differential transmission-lines that are resistant to the EMI and less susceptible to ground voltage disturbance are therefore widely used. However, in order to contain complicated circuit structures in a limited circuit layout, the differential transmission-lines must have turning points or corners. The turning points or corners often make the differential transmission-lines have unequal lengths so as to generate common-mode noises.
Therefore, some methods for suppressing the common-mode noises have been proposed. For example, the common-mode chock is the most typical one. The common-mode chock is made by winding two independent coils having the same number of turns onto a magnet. The common-mode chock is structurally equivalent to a winding magnetic core, can generate a high conductivity impedance to common-mode noises, and can generate an impedance of almost zero to differential mode signals. However, common-mode chokes usually only work in the MHz range. Because of the frequency characteristics and the parasitic effect of ferromagnetic materials, the common-mode chokes are difficult to be designed at high frequencies over GHz. In addition, the complicated structures of the common-mode chokes are less suitable for miniaturization of circuit nowadays.
Accordingly, a kind of common-mode filters using patterned ground structures to eliminate common-mode noises have been proposed, which can work in the GHz range and have lower costs. However, because the sizes of the patterned ground structures must be half or quarter of the wavelength of transmission signals, the patterned ground structures will occupy a large ground area of a printed circuit board, so that the common-mode filters can not effectively reduce the production areas.
To sum up, how to provide a common-mode filter to solve the shortcomings of too large areas and high frequency operation inefficiencies in the prior arts becomes an important issue to be solved by those in the industry.